3GPP RANI WG has approved a study item “Study on Downlink Multiuser Superposition Transmission for LTE” (RP-151100) for studying Non-Orthogonal Multiple-Access (NOMA) for downlink, a technique for multiuser transmission. When coupled with a Successive Interference Cancellation (SIC) receiver at the User Equipment (UE) side, the NOMA technique is expected to achieve a capacity region in a multi-user environment.
FIGS. 1a and 2b show basic principles of the NOMA technique, according to which two or more UEs share the same resource, i.e. time-frequency slot(s), and the UEs are assigned with unequal powers. In the case where only two UEs are coupled, they may be referred to as near-user equipment (UEN) 104 and far-user equipment (UEF) 106. Ignoring the propagation channel, the received signal r of either of the UEs is given by the following equation—r=√{square root over (αN)}sN+√{square root over (αF)}sF+n Where sN and sF are the modulation symbols sent to UEN and UEF respectively and n is the noise. αN and αF are the normalized power ratios for the UEN 104 and the UEF 106 respectively and satisfy the following condition:αN+αF=1
A multi-user receiver is a receiver configured to demodulate mutually interfering streams of signals, each stream of signals being intended for a different user. In contrast, a single-user receiver is configured to demodulate signals intended for a single user without explicitly demodulating signals intended for other users.
In FIG. 1, because of the UEN 's relative proximity to the base station (or eNode in an LTE network) 102, the signals intended for the UEN 104 may be assigned with a smaller amount of power compared to that assigned for the signals intended for UEF 106. Hence the UEN typically experiences relatively strong interference from the signals intended for UEF.
In the NOMA technique, data packets intended for multiple users are sent within an LTE subframe over same physical resources. In the case of FIG. 1, a data packet intended for UEN 104 share the same physical resources with data packet intended for UEF 106. The UEN 104 will then need to use a multi-user receiver, which is able to extract the data/signals intended for the UEN 104 out of the received signal r.
The UEF 106 on the other hand, receives signals intended for the UEN 104 as weak signals and can often treat them as background noise. Hence the UEF may adopt a single-user receiver to directly decode its own signal out of the received signal r.
For the UEN 104 to correctly detect and filter out the signals intended for UEF 106, preferably it is configured to be familiar with the physical layer parameters of the transmission of UEF 106. These parameters are dynamically configured and included in a Downlink Control Indication (DCI) message intended for the UEF, which is referred to as DCIF. How a first UE (e.g. UEN) could acquire the DCI information of a second UE (e.g. UEF), which is paired with the first UE for the purpose of multi-user transmission has not been addressed so far.
The embodiments of the present invention provide improved techniques for Non-Orthogonal Multiple-Access (NOMA) in downlink multi-user transmission primarily in an LTE network.
The embodiments described below are not limited to implementations which solve any or all of the disadvantages of known systems.